A solenoid is a device which converts electrical energy into mechanical movement. It consists primarily of two parts, a coil and an armature. Generally, the coil is formed from wire which has been wound into a cylindrical shape. The armature is mounted to move or slide axially within the cylindrically shaped coil. An electrical signal applied to the coil generates an electromagnetic field. The electromagnetic field causes the armature to move axially within the coil.
The coil and armature are usually mounted in a casing. The structure of the casing and armature are designed to limit the amount of armature movement, i.e., to limit stroke distance. Typically, when an electrical signal is applied to the coil and the resulting electromagnetic field causes the armature to move, a plate attached to the armature strikes the casing, stopping armature movement.
Solenoids of various types have been used for many years to operate mechanisms requiring some form of lateral, vertical, horizontal or axial motion. In some instances, solenoids are used to control mechanisms that contain precharged forces, i.e., mechanisms which retain tensioned springs, pressurized air or pressurized oil. The precharged forces are released from such mechanisms by valving or latching systems. Typically, the valving or latching systems are controlled by the motion of a solenoid. Consequently, applying an electrical signal to the solenoid will result in the release of the precharged force. An example of such a precharged mechanism is a circuit breaker found in power distribution systems.
In many instances, circuit breakers are hydraulically controlled, i.e., a hydraulic fluid is used to cause movement in the circuit breaker to break or make electrical contact, i.e., to open or close the circuit breaker. Pressurized hydraulic fluid is passed through a changeover valve. Depending on the position of the valving components in the changeover valve, pressurized fluid will be applied to the circuit breaker causing it to either open or close. For such circuit breakers, the valving or latching systems must act very quickly, i.e., on the order of 30-60 milliseconds. In the past, solenoids have been developed which exhibit movement to adequately achieve the necessary valving or latching speed.
However, recent developments in the area of power distribution systems and in the electronic designs used in such systems, have created a need to operate circuit switching devices much more accurately. For example, some present designs will attempt to either open or close circuit switching devices at particular points within an alternating current (AC) cycle, i.e., at particular points on the AC sine wave. In such applications, accuracy, under all operating conditions, to within a few tenths of a millisecond is needed to make such schemes practical. Such accuracy is not possible with traditionally designed solenoid assemblies.
Moreover, the valving and latching systems which control operation of circuit switching devices tend to vary their response to solenoid control in relation to the amount of time which has passed since the last solenoid movement. Using the above example, changeover valves may be harder to open or close depending on how long the valving components have maintained their present state. In such cases, a greater force is needed to operate the valve system. In other words, the force necessary to operate the changeover valve will vary over time.
These two circumstances, namely, the timing required by contemporary circuit designs and the varying over time of the force required to effectuate operation, leads to unacceptably high inaccuracies using traditional solenoid assemblies.
Consequently, a need still exists for a solenoid assembly which is capable of generating sufficient force at a precise time.